Various types of devices for detecting an electromagnetic wave using an electro-optic crystal and the like have been developed. As an example of such a technology for detecting an electromagnetic wave, for example, Patent Document 1 (JP 2005-214892A) discloses the following technology. That is, an electric field sensor including: a sensor head unit with an electro-optic crystal; a signal processing unit configured to detect a measurement target electric field based on an output of the sensor head unit; a polarization maintaining fiber configured to transmit light from the signal processing unit to the sensor head unit so that the light enters a first surface of the electro-optic crystal; and an optical fiber configured to transmit, to the signal processing unit, light that has propagated through the electro-optic crystal and has exited from a second surface of the electro-optic crystal that is opposite to the first surface, wherein the signal processing unit is provided with: a light source that emits linear polarized light so that the linear polarized light is transmitted to the sensor head unit by the polarization maintaining fiber; a linear polarized light generator that converts the light transmitted by the optical fiber into linear polarized light that has a polarization plane at a preset angle; a polarization separation element that separates the light output from the linear polarized light generator into S polarized light and P polarized light; a first light detector that photoelectrically converts the S polarized light; a second light detector that photoelectrically converts the P polarized light; a differential amplifier that subjects an output electric signal of the first light detector and an output electric signal of the second light detector to differential amplification; and an electric signal measuring apparatus that detects a measurement target electric field based on the output electric signals of the differential amplifier, wherein the angle of the polarization plane of the linear polarized light that enters the first surface of the electro-optic crystal is about 45 degrees with respect to either one of two electric principal axes of the electro-optic crystal, and the linear polarized light generator has a response speed that is lower than a lower limit of the frequency of the measurement target electric field.
Furthermore, Patent Document 2 (JP 2001-343410A) discloses the following technology. That is, an electric field probe including: an electro-optic crystal that is sensitive only to an electric field that is parallel or orthogonal to a traveling direction of laser light; a laser light emitting means for emitting laser light to the electro-optic crystal; an optical polarization detection means for receiving the laser light from the laser light emitting means that entered and was reflected off the electro-optic crystal, and converting a polarization change of the laser light into an intensity change of the laser light; a light detection means for converting the intensity change of the laser light converted by the optical polarization detection means into a detection signal indicating the intensity change of the electric signal; and an output means for outputting an intensity and phase of the detection signal detected by the light detection means.
Furthermore, Patent Document 3 (JP 2014-52272A) discloses the following technology. That is, an electromagnetic wave detection system including: a first light source; a second light source that emits light at a frequency different from that of the first light source; a first optical branching device that branches the light from the first light source into two; a second optical branching device that branches the light from the second light source into two; a first optical multiplexer that multiplexes one of the two streams of light branched by the first optical branching device and one of the two streams of light branched by the second optical branching device; a second optical multiplexer that multiplexes the other one of the two streams of light branched by the first optical branching device and the other one of the two streams of light branched by the second optical branching device; an oscillator that outputs an oscillation signal at a constant frequency; a frequency shifter that is interposed between the first optical branching device and the first optical multiplexer, between the second optical branching device and the first optical multiplexer, between the first optical branching device and the second optical multiplexer, or between the second optical branching device and the second optical multiplexer, and uses the oscillation signal from the oscillator to shift the frequency of light by the frequency of the oscillation signal; an RF signal generator that receives two types of light having different frequencies from the first optical multiplexer, generates a first RF (Radio Frequency) signal with a frequency equal to the difference between the two types of light, and emits the generated first RF signal to a measurement target; an RF signal detector that mixes the first RF signal that is transmitted through or reflected by the measurement target with LO (Local Oscillating) signals, which are the two types of light having different frequencies from the second optical multiplexer, subjects the resultant signal to heterodyne detection, and outputs a detection signal with a frequency equal to the difference between a frequency equal to the difference between the two types of light constituting the LO signals, and the frequency of the first RF signal; and a 2 phase lock-in detector that subjects the detection signal output by the RF signal detector to 2 phase lock-in detection using the oscillation signal output by the oscillator, and outputs an in-phase component that is in phase with the detection signal output by the RF signal detector, and an orthogonal component that has a phase orthogonal to the detection signal output by the RF signal detector.